1. Field of the Invention
The present invention relates generally to an active suspension system for an automotive vehicle. More specifically, the invention relates to a working fluid circuit for the active suspension system which prevent vehicular height level from suddenly varying upon initiation of suspension control operation.
2. Description of the Background Art
Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-219408 discloses an active suspension system. The active suspension system, in the prior proposal, has a surge control valve in a supply line of a hydraulic circuit and a flow control valve in a drain line. The flow control valve is responsive to the fluid pressure supplied to the supply line dropping below a predetermined set pressure for blocking fluid flow through the drain line. The flow control valve is associated with a fluid pressure regulation means regulating fluid pressure in the drain line upstream of the flow control valve. The flow control valve and the fluid pressure regulating means are cooperated for maintaining the working fluid pressure in a working chamber of an active cylinder at a predetermined level. Such working fluid circuit construction is effective for preventing the vehicular height from suddenly lowering when control operation is terminated to cause lowering of the fluid pressure to be supplied to the supply line.
However, even by the flow control valve and the fluid pressure regulating means, it is not possible to completely prevent the working fluid from lowering due to leakage of the working fluid and/or to reduction of volume of the working fluid which can be caused by lowering of temperature. This necessarily cause lowering of the fluid pressure in the working chamber and thus results in lowering of the vehicular height from a desired height level.
On the other hand, Japanese Patent First (unexamined) Publication (Tokkai) Showa 63-328499 proposes a working fluid circuit provided therewith a pressure accumulator for absorbing back pressure in a drain line. Absorption of the back pressure in the drain line may reduce resistance of the fluid flow in the drain line and thus improve response characteristics in pressure control in the working chamber. On the other hand, the pressure accumulator tends to serve for lowering fluid pressure below the predetermined pressure level while the drain line is blocked by continuously absorbing the fluid pressure in the closed circuit. This results in sudden variation of the vehicular height level upon resumption of the line pressure, such as upon restarting of the suspension control operation. This clearly cause degradation of the riding comfort.
For improving this, it may be an approach to be taken to adjust the increasing rate of the fluid pressure in the working chamber by means of a pressure control valve. However, upon resumption of the line pressure, the back pressure in the drain line may serve to cause rapid increasing of the output pressure in the working chamber. Therefore, this approach cannot be successful in preventing the vehicular height from causing sudden variation.